Lateral MEMS-Type Field Emission Electron Source - IEEE Xplore

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 63, NO. 2, FEBRUARY 2016

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Lateral MEMS-Type Field Emission Electron Source Tomasz Grzebyk, Piotr Szyszka, Anna Górecka-Drzazga, and Jan A. Dziuban

Abstract— This paper describes a microelectromechanicalsystem-type field emission electron source fabricated as a planar silicon structure bonded with a glass substrate. It consists of a carbon nanotube cathode, beam formation electrodes, and silicon glass vacuum housing, all made in a uniform technological process. The current–voltage characteristics obtained inside a reference vacuum chamber for the two-, three-, and fourelectrode configurations have been presented. The possibility of generation of a focused electron beam as well as gas ionization has been investigated. In addition, the lateral electron source has been integrated on the chip with a miniature ion-sorption vacuum pump and hermetically sealed. The use of the micropump significantly improved the stability of field emission current. Index Terms— Electron source, field emission, ion-sorption micropump, microelectromechanical-systems (MEMS).

I. I NTRODUCTION OST of the vacuum nanoelectronic devices consist of three major elements: 1) an electron source; 2) a beam formation electrode system; and 3) a vacuum housing [1]. In miniature devices, thermal cathodes have been replaced by cold cathodes, which are usually fabricated by the use of microelectronics and microengineering techniques [2], [3]. Recently, they are also often made of different kinds of nanomaterials, e.g., carbon nanotubes (CNTs) [4]–[8]. The extraction gate and focus electrodes can be integrated with the cathode on the same chip [9] or can be manufactured separately and placed just above the electron source [10]. Some complex vacuum instruments like miniature mass spectrometers [11], free electron lasers [12], and terahertz sources [13], [14] need a large number of additional electrodes, which are specially designed to enable, for example, ionization of gases or generating electromagnetic signals. The last but one of the most important elements of a complete vacuum electronic device is a vacuum-tight housing. Since field emission cathodes are very fragile and sensitive to the presence of residual gases, they require high and stabile vacuum for proper operation [15].

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Manuscript received October 29, 2015; revised December 1, 2015; accepted December 4, 2015. Date of publication December 24, 2015; date of current version January 20, 2016. This work was supported in part by the Polish National Science Center under Grant 2013/09/B/ST7/01602 and in part by the Statutory Grant of Wrocław University of Technology under Grant S 400 74 Z12/Z7. The review of this paper was arranged by Editor G. L. Snider. (Corresponding author: Tomasz Grzebyk.) The authors are with the Wrocław University of Technology, Wrocław 50-372, Poland (e-mail: [email protected]; piotr. [email protected]; [email protected]; jan.dziuban@pwr. edu.pl). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2506778

Fig. 1. Lateral field emission electron source. (a) Cross section of the device. (b) 3-D visualization. (c) Photograph of the fabricated structure.

In many scientific papers, authors concentrate on one of three mentioned elements and it is not taken into an account that they should create together a complete vacuum system. In this paper, we present a way to fabricate vacuum nanoelectronic devices by taking benefit of microelectromechanicalsystems (MEMS) technology, which allows integration of a field emission cathode, an electron beam forming electrodes, and a vacuum housing on one chip. II. D ESIGN AND C ONSTRUCTION Recently, we have demonstrated the first approach to realize this concept and have shown a self-packaged MEMS-type field-emission electron source [16]. It was fabricated as a stack of vertically aligned silicon electrodes and glass spacers connected by the use of an anodic bonding process. This time, we present a device in which all the electrodes of the field emission electron source (cathode, extraction electrode, focus electrode and anode) are formed in a single silicon wafer anodically bonded with a glass substrate (Fig. 1). Independently of the number of electrodes, the fabrication process requires only one photolithography step. The size and spacing between the electrodes depend only on the pattern of the photolithographic mask and on the applied silicon etching process (wet or dry method). In this project, we have used wet anisotropic etching of silicon, and the smallest distance between the electrodes was equal to 100 μm. To ensure good field emission properties of the device, the CNTs were deposited onto the tip of the silicon cathode (without the CNT layer, no emission current was observed below 2000 V).

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Fig. 2. Basic steps in the lateral field emission electron source fabrication process.

Back-side contacts to the silicon electrodes were realized through metalized holes made in the glass substrate. The electron source was hermetically encapsulated by the use of a top glass cover. The presented approach is very flexible and allows fabrication of different types of vacuum nanoelectronic devices in a relatively simple way. III. T ECHNOLOGY The test structures were made using a 3-in, (100) oriented, 400-μm-thick, double-side polished, high conductive (0.001–0.01 cm) n-type silicon wafer (Institute of Electronic Material Technology, Poland) and a 1.1-mm-thick borosilicate glass substrate (Borofloat 3.3, Schott, Germany). First, silicon was thermally oxidized (2) and two SiO2 masks, aligned together at both sides of the substrate (3), were photolithographically formed (4) (Fig. 2). Next, silicon wafer was etched from both sides in 10M KOH water solution at 80 °C (5). When a membrane supporting all the elements was 50 μm thick, the etching process was stopped, and bottom oxide layer was removed (6). Next, the silicon chip was anodically bonded with the glass substrate (V = 1500 V, T = 400 °C, t = 15 min) (7). The etching process of the silicon was continued until the electrodes were completely separated (8). The cathode, the focus electrodes, and the anode formed completely independent islands connected with the glass substrate (9). The extraction gate formed a frame that surrounded the other electrodes. At this stage of the fabrication process, 0.7-mm-diameter holes were made in glass substrate (10) and a 0.5-μm-thick aluminium layer was magnetron sputtered onto the bottom side of the glass substrate, to make back-side electrical contacts to the electrodes (11). Commercially available single-walled CNTs (CheapTubes, USA, 1–2 nm wide and 30 μm long) were deposited onto the tip of the cathode by the use of electrophoretic process (200 V, 3 min) (12). This method ensures random distribution and orientation of individual nanotubes. The top glass cover was etched in 40% HF, hydrophilizated, and anodically bonded with the silicon frame in vacuum conditions to obtain a hermetic package (13).

Fig. 3. Field-emission characteristics for the two- and three-electrode configurations. (a) Cathode current (IC ) versus gate-cathode voltage. (b) Gate current (IG ) and anode current (I A ) versus anode–cathode voltage, VGC = 800 V.

IV. R ESULTS AND D ISCUSSION First, properties of the electron source, without the top glass cover, have been examined for the two-, three-, and four-electrode configurations in the reference vacuum chamber ( p = 3 × 10−5 hPa). Field emission current flowing from the cathode to the gate reached 65 μA for VCG = 1000 V (the anode was not polarized) [Fig. 3(a)]. For the three-electrode configuration with increasing anode–cathode voltage, the gate current dropped, and the anode began to capture emitted electrons [Fig. 3(b)]. Transmission ratio (I A /IC ) reached 84% when the anode–cathode voltage was higher than the gatecathode voltage. Addition of the fourth electrode allowed more precise steering of the emitted electron beam. First of all, its potential influenced the cathode and the anode current, which increased significantly with increasing VFC [Fig. 4(a)]. Moreover, it was possible to control the width of the beam of the emitted electrons. It was confirmed by the SIMION simulations and also by the measurements. For VFC < 700 V (VGC = 1200 V and VAC = 1500 V), electrons were pushed away from the focus electrode [Fig. 4(b)]. It can also be noted on a colorful trace left on the glass substrate after the measurements [Fig. 4(c)]. In this case, I F current had a negative value, which means that the focus electrode attracted positive ions generated by the electrons colliding with residual gases. When VFC > 700 V, the focus electrode draws part of the electrons

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Fig. 5. (a) Vacuum encapsulated electron source and (b) glow discharge inside the device ( p ∼ 0.1 hPa).

Fig. 4. Measurements of the four-electrode configuration. (a) Field-emission current as a function of the focus-cathode voltage. (b) Simulation of distribution of electric field lines for VAC = 1500 V, VGC = 1200 V, and VFC = 400 V. (c) Photograph of the structure after the measurements.

and acts as an additional gate, increasing the potential near the cathode tip. During the further experimental stage, the field emission electron source has been examined after hermetic encapsulation. It is known from the literature [15], [17] as well as from our previous investigations [16], [18] that vacuum conditions have a significant influence on the field emission properties. Moreover, it was proved that although anodic bonding is reliable and the leakage between the sealed silicon and glass layers is very small, this process is insufficient to ensure stable high vacuum conditions inside a structure in long time period [19], [20]. It is mostly related to the presence of the strong outgassing during anodic bonding performed at high temperature, and later to degassing from internal surfaces, and also to permeation of light gases through the materials [21]. In order to reduce these phenomena, the nonevaporable getters are often used in different vacuum microsystems. Also in this experiment, immediately before the vacuum anodic bonding, the getter (SEAS Getters, St 212) was introduced into the silicon glass structure. Sealing process was performed at p = 1 × 10−3 hPa for 45 min at 450 °C. These conditions should be sufficient to activate the getter and to improve vacuum. However, it turned out that vacuum inside the encapsulated device was too low. At UAC = 500 V, instead of field emission current, a glow discharge of the residual gases was observed (Fig. 5). Similar effects were obtained in the reference vacuum chamber at pressure as high as 1–10 × 10−2 hPa. It means that the getter did not allow to generate high vacuum. It seems that the only possible way to generate high vacuum inside the MEMS structure is to use miniature vacuum pump,

Fig. 6. Lateral field emission electron source integrated with the miniature vacuum pump. (a) Schematic cross section. (b) Photograph of the integrated device without the magnets.

which is elaborated recently in [22]. With its application, pressure can be reduced to a value lower than 1 × 10−5 hPa. Micropump could be fabricated separately and placed together with the electron source in an external vacuum-tight housing, but in this research, an approach to integrate both devices on the same chip was made (Fig. 6). A new silicon chip was designed. A square window was added to the silicon layer next to the electrodes of the electron source. This electrode worked as an anode of the micropump. In addition, the openings in the bottom and top glass wafers forming a micropump cavity had to be made. Two additional silicon substrates have been used as cathodes of the micropump. Magnetic field was introduced into the micropump structure by two external neodymium magnets. All the layers were bonded step by step and the last process was performed, previously, in vacuum conditions to ensure initial vacuum to turn ON the micropump. A glow discharge inside the micropump cavity ignited when 600 V was applied between the cathode and the anode of the micropump. At this moment, the ion current had a value of 43 μA. According to the calibation curve (relation between the discharge current and the pressure), it corresponded

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Fig. 7. Stability of the field emission electron current versus time operation for the integrated device when the micropump was turn OFF (orange line) and turn ON (blue line).

to 9 × 10−3 hPa [22]. During the pumping, discharge current dropped very quickly (after 2 s) to a value of 4 μA, which corresponds to pressure of 1 × 10−3 hPa. A further increase in voltage to 700 V enabled to reduce pressure to 5 × 10−4 hPa (after 5 s) and to 2 × 10−4 hPa (after 1 minute). Finally, when 1200 V was applied, the pressure of 6×10−6 hPa was achieved after 5 min. At this stage, the micropump was turned OFF and field emission current measurements started. No glow was observed, but electron current stability was poor. Fluctuations reached 50% and after 10 min, the current dropped to 20% of the initial value (Fig. 7, orange line). To improve the current stability, a next step forward was made—emission current was measured during the micropump operation. When micropump was turned ON once again, the measured discharge current corresponded to internal pressure of about 1 × 10−3 hPa. It was an in situ evidence that pressure during the operation of the electron source gradually increased, probably due to outgassing of the emissive layer and bombardment of the surface of electrodes by electrons and ions [23]. Next, the integrated device was pumped to 6 × 10−6 hPa (about 5 min) and study of the electron source was continued with a pump turned ON (Fig. 7, blue line). This time, current fluctuations were significantly reduced to 10%, and after 10 min of operation, almost no degradation effects were noticed. The only disadvantage in this case is the fact that there might, by some influence of the magnetic field, be generated by the magnets used in the micropump on the electron beam. If so, there is a simple way to prevent it— using a clamp made of a magnetic material, which connects the external poles and restricts the field only to the pump area. V. C ONCLUSION The lateral CNT field emission electron source has been fabricated by the use of MEMS technology. Compared with the previously reported structure, it has some positive and negative sides. The technological process is a bit more complex (e.g., necessity of making electrical feedthroughs) but on the other hand is more scalable. The technological complexity is the same for the structure with two and ten electrodes. There is also no problem with the alignment of individual electrodes

and with multiple-anodic-bonding process as it was in the vertical source. The lateral source showed satisfactory field emission properties during the measurements in a reference vacuum chamber. The possibility of steering the electron beam at high vacuum and the ability of gas ionization at low vacuum have been proven. The electron source after vacuum encapsulation did not exhibit good performance. It was found that the pressure in the microcavity was too high to enable field emission of electrons, even though getter was applied. A significant improvement was achieved after integration of the electron source with a miniature ion-sorption vacuum pump. The experiments revealed that to ensure stable vacuum conditions inside the integrated device, the micropump has to work continuously during the electron source operation. The elaborated method of integration of MEMS devices can find application in a construction of different vacuum nanoelectronic devices, for example, in a miniature mass spectrometers. R EFERENCES [1] W. Zhu, “Field emission flat panel displays,” in Vacuum Microelectronics. New York, NY, USA: Wiley, 2001, pp. 289–348. [2] C. A. Spindt, “A thin-film field-emission cathode,” J. Appl. Phys., vol. 39, no. 7, pp. 3504–3505, Feb. 1968. [3] D. Temple, W. D. Palmer, L. N. Yadon, J. E. Mancusi, D. Vellenga, and G. E. McGuire, “Silicon field emitter cathodes: Fabrication, performance, and applications,” J. Vac. Sci. Technol. A, vol. 16, no. 3, pp. 1980–1990, 1998. [4] N. S. Xua and S. E. Huq, “Novel cold cathode materials and applications,” Mater. Sci. Eng. R, Rep., vol. 48, nos. 2–5, pp. 47–189, 2005. [5] M. Yumura et al., “Synthesis and purification of multi-walled carbon nanotubes for field emitter applications,” Diamond Rel. Mater., vol. 8, nos. 2–5, pp. 785–791, 1999. [6] C. Li et al., “Structural, photoluminescence, and field emission properties of vertically well-aligned ZnO nanorod arrays,” J. Phys. Chem. C, vol. 111, no. 34, pp. 12566–12571, 2007. [7] W. I. Milne et al., “Carbon nanotubes as field emission sources,” J. Mater. Chem., vol. 14, no. 6, pp. 933–943, 2004. [8] G. S. Bocharov and A. V. Eletskii, “Theory of carbon nanotube (CNT)-based electron field emitters,” Nanomaterials, vol. 3, no. 3, pp. 393–442, 2013. [9] Y. Neo et al., “Emission and focusing characteristics of volcanostructured double-gated field emitter arrays,” J. Vac. Sci. Technol. B, vol. 27, no. 2, pp. 701–704, 2009. [10] M. Despont, U. Staufer, C. Stebler, R. Germann, and P. Vettiger, “Microfabrication of lenses of a miniaturized electron column,” Microelectron. Eng., vol. 27, nos. 1–4, pp. 467–470, 1995. [11] L. F. Velásquez-García, B. L. P. Gassend, and A. I. Akinwande, “CNT-based MEMS/NEMS gas ionizers for portable mass spectrometry applications,” J. Microelecromech. Syst., vol. 19, no. 3, pp. 484–493, Jun. 2010. [12] F. Floreania, H. W. Koops, and W. Elsäßer, “Concept of a miniaturised free-electron laser with field emission source,” Nucl. Instrum. Methods Phys. Res. A, Accel., Spectrom., Detect. Assoc. Equip., vol. 483, nos. 1–2, pp. 488–492, 2002. [13] H. Ishizuka, Y. Kawamura, K. Yokoo, H. Shimawaki, and A. Hosono, “Smith–Purcell experiment utilizing a field-emitter array cathode: Measurements of radiation,” Nucl. Instrum. Methods Phys. Res. A, Accel., Spectrom., Detect. Assoc. Equip., vol. 475, nos. 1–3, pp. 593–598, 2001. [14] H. W. P. Koops, A. Al-Mudhafar, and H. L. Hartnagel, “Miniaturized THz source with free-electron beams,” in Tech. Dig. 24th IVNC, Jul. 2011, pp. 187–188. [15] K. A. Dean and B. R. Chalamala, “The environmental stability of field emission from single-walled carbon nanotubes,” Appl. Phys. Lett., vol. 75, no. 19, pp. 3017–3019, 1999. [16] T. Grzebyk, A. Górecka-Drzazga, and J. A. Dziuban, “MEMS-type selfpackaged field-emission electron source,” IEEE Trans. Electron Devices, vol. 62, no. 7, pp. 2339–2345, Jul. 2015.

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[17] Y. Zhang, S. Z. Deng, N. S. Xu, and J. Chen, “Study of influence of gas ambience on the CNT cold cathode property in high temperature process,” in Tech. Dig. 20th IVNC, Jul. 2007, pp. 221–222. [18] T. Grzebyk and A. Górecka-Drzazga, “Miniature ion-sorption vacuum pump with CNT field-emission electron source,” J. Micromech. Microeng., vol. 23, no. 1, p. 015007, 2013. [19] M. Esashi, “Wafer level packaging of MEMS,” J. Micromech. Microeng., vol. 13, no. 7, p. 073001, 2008. [20] J. Chae, J. M. Giachino, and K. Najafi, “Fabrication and characterization of a wafer-level MEMS vacuum package with vertical feedthroughs,” J. Microelectromech. Syst., vol. 17, no. 1, pp. 193–200, Feb. 2008. [21] T. Grzebyk and A. Górecka-Drzazga, “Vacuum microdevices,” Bull. Polish Acad. Sci., Tech. Sci., vol. 60, no. 1, pp. 19–23, 2012. [22] T. Grzebyk, A. Górecka-Drzazga, and J. A. Dziuban, “Glow-discharge ion-sorption micropump for vacuum MEMS,” Sens. Actuators A, Phys., vol. 208, pp. 113–119, Feb. 2014. [23] E. O. Popov and A. G. Kolosko, “Field emission and gases desorbtion of MWCNT emitters,” in Tech. Dig. 25th IVNC, Jul. 2015, pp. 1–2.

Piotr Szyszka received the M.Sc.Eng. degree from the Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wroclaw, Poland, in 2014, where he is currently pursuing the Ph.D. degree.

Anna Górecka-Drzazga received the M.Sc. and Ph.D. degrees in electronics and the D.Sc. degree in electronics with a minor in microsystems from the Wrocław University of Technology, Wrocław, Poland, in 1974, 1978, and 2008, respectively. She has been a Professor with the Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, since 2009. Her current research interests include MEMS, Micro-OptoElectro-Mechanical Systems, micro Total Analysis Systems microsystems, and vacuum nanoelectronics devices.

Tomasz Grzebyk received the M.Sc.Eng. and Ph.D. degrees from the Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wrocław, Poland, in 2009 and 2014, respectively, and the M.Sc. degree from the Faculty of Physics and Astronomy, Wrocław University, Wrocław, in 2011. He has been an Assistant with the Wrocław University of Technology since 2014.

Jan A. Dziuban received the M.Sc. and Ph.D. degrees in electronics from the Wrocław University of Technology, Wrocław, Poland, in 1974 and 1978, respectively. He has been a Professor and the Head of the Department of Microengineering and Photovoltaics with the Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, since 2005.